Half-tone image recording system

ABSTRACT

In an image-reproducing system in which a number of parallel light beams contribute to the formation of half-tone dot areas on a record medium and scan over the record medium together, a reversing prism is included in the beam path to permit the line fit or line overlap to be closely controlled. The prism also adjusts the spacing between beams in the scanning direction which is provided when a coherent light source is used to avoid interference between adjacent beams.

This invention relates to image reproducing systems and especially tohalf-tone image reproducing systems of the kind in which a number ofseparate light beams are simultaneously scanned over an image-recordingsurface, the light beams falling on points such that some or all of themcontribute to the formation of a single half-tone dot area. Beamscontributing to a single half-tone dot area may be modulated with thesame or with different image information for that area and are modulatedwith different half-tone information. If the lines recorded by differentbeams are to be adjacent one another, it is important to ensure thatthey are accurately positioned so as to avoid undesired overlapping orthe creation of "empty" lines between the scanned lines. Alternatively,it may be desired for adjacent beams to overlap to provide a desiredintensity pattern, as disclosed in U.S. Pat. No. 4,025,189 dated May 24,1977 and in this case it is important that the predetermined degree ofoverlap is maintained in order to avoid undesired intensity variations.

A half-tone image recording system according to the present inventioncomprises: means for generating a number of separate beams of radiationwhich, in cross section, form a line of spaced beams; a support for arecording surface to be treated by exposure to the beams of radiation;means producing relative movement of the recording surface on the onehand and the beams on the other hand, whereby the recording surface isscanned by the beams; modulating means for individually modulating thebeams incident at points on the recording surface as required by thedensity variations of the image to be reproduced and by the screendensity variations required to produce a half-tone image, the modulationbeing such that some or all of the modulated beams contribute towardsthe recording of a single half-tone dot; and means for rotating the lineof beams about an axis parallel to their direction of travel to alterthe angle of the line of beams incident on the recording surfacerelative to the direction of beam scanning over the recording surface.

Preferably, the means for rotating the line of beams is a reversionprism (K prism). However, other forms of reversing prisms may be used,for example a Dove prism; such prisms are sometimes called half-speedprisms because as the prism is rotated around the optical axis, an imagerotation of twice the amount of prism rotation is produced. An advantageof such prisms is that the axis along which the light emerges from theprism is a continuation of the axis along which it enters the prism.

Thus, the angle of inclination of the spots on the image recordingsurface is not dependent on the orientation of the original row ofbeams. Rotation of the prism provides a fine adjustment for ensuringthat the line scanned by each spot is correctly positioned in relationto the lines scanned by adjacent spots. In other words, the extent towhich the lines fit against one another, or overlap one another, may becontrolled. These adjustments are obtained without the rotation of thewhole of the optical system which would be necessary in the absence ofthe reversion prism.

Additionally, the spacing between beams in the direction of scanning isadjusted. By providing a spacing between the beams in the scanningdirection it can be ensured that two adjacent beams do not fall on thesame recording area at the same time. This is important when the beamsare of coherent radiation and are derived from a common laser, sinceoverlapping of the beams would then produce interference which woulddistort the tone values of the image. By suitably positioning the beamsin relation to one another it can be ensured that the beam tracks fitagainst one another or overlap one another, as required, withoutallowing adjacent beams to fall on the same spot at the same time. Whenthe beams are spaced in the direction of scanning in this way, themodulation signals controlling the later beams are delayed in accordancewith the spacing between the beams at the recording surface.

In order that the invention may be better understood, one systemembodying the invention will now be described, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 shows image-reproducing means embodying the invention;

FIG. 2 illustrates the optical system of FIG. 1, including a reversionprism; and

FIG. 3 illustrates the shape and arrangement of the apertures of themask of FIG. 2.

In the image-reproducing apparatus shown in FIG. 1, a motor 10 drives aninput drum 12, an output drum 14 and a shaft transducer 16 mounted on acommon shaft. The input drum 12 carries the image 13 to be reproducedand this image is scanned by an analysing head 20. The output drum 14carries a film 15 to be exposed to form a monochrome colour separationfor a selected colour component. The film 15 is scanned by themultiple-output exposing head 22. These two heads are mechanicallycoupled through a member 24 and move together along a lead screw 26controlled by a traverse servo 28 in turn controlled by the signals fromthe shaft transducer 16.

The output of the scanning head 20 is in the form of three colourchannel signals (red filter, blue filter and green filter) and thesesignals are fed to a colour computation unit 30 in which corrections areintroduced in a manner known in itself, the value of the correctingsignal for one channel depending at least partially upon the values ofthe signals in the other channels. At the output of the colourcomputation unit 30, one of the three corrected colour signals is chosenby means of a colour-component selector switch 32 and is applied to amultiple-signal generator 34. This generates six signals which, in thisexample, are composed of the common signal from the colour computationunit 30 each in combination with a different screen-representing signal.The resulting screened and corrected colour signals are applied througha delay unit 36 to an optical system 38 in the exposing head 56. Theoptical system 38, which will be described in greater detail withreference to FIG. 2, includes individual modulators for six beamsderived from a laser 40 by means of a beam splitter 42. The modulatedbeams are incident on the film 15 to be treated on the cylinder 14.Together, the six light beams contribute to the making of the half-toneexposure dots in the colour separation. They may contribute to twoadjacent dots at any instant.

An image-reproducing system including a screened signal generatoremploying combinations of shift registers and signal generating units isdescribed in U.S. Pat. No. 4,012,584. Such a screened signal generatormay be used in the reproducing system described above.

In FIG. 2, the laser 40, which may be a 10 mw Argon laser(Spectra-physics Model 162) operating at 488 nm, generates a coherentlight beam which is applied to the beam splitter 42. In this example,the beam splitter 42 splits the original beam into six parallel beamswhich are applied through a half-wave plate 44 to a modulator system.The modulator system comprises an input polariser plate 46 and an outputpolariser plate 50 separated by electro-optic modulators 48, by means ofwhich each of the light beams can be separately modulated. The modulatedbeams pass through prisms 52 and 54 which serve for anamorphic beamcompression and the resulting closely spaced beams fall on a mask 56.The mask 56 contains six apertures defining the shape required for thebeam cross-sections at the image-recording surface. Beams passingthrough the mask 56 (represented for simplicity by a single beam in thedrawing) are collected by a field lens 58 and are reflected by a mirror60 to a prism 62. The prism 62 is of the kind known as a "reversionprism". If the grooved surface of the prism is considered as its top, abeam incident on the prism is reversed in the top-to-bottom sense (180°rotation) but not in the lateral sense (0° rotation). A vector in anintermediate position is rotated by an intermediate amount.

Light leaving the reversion prism 62 passes through a negative lens 64and a focusing lens 66 before falling on the film 15 on the surface ofthe cylinder 14.

In U.S. Pat. No. 4,025,189 dated May 24, 1977 there is described animage-reproducing system in which a record medium is exposed by a row oflight modulators and in which the areas of illumination of the recordmedium by adjacent tones of the individual light modulators overlap. Theoptical apparatus shown in FIG. 2 is intended for use in such a system.For this reason, the apertures of the mask 56 are of the form shown inFIG. 3.

FIG. 3 illustrates the pattern and shape of the apertures on the maskand also the pattern and shape of the beam spots as they fall on thefilm 15. The arrow 68 represents the direction of rotation of thecylinder and the arrow 70 represents the direction of slow traverse ofthe scanning head with respect to the cylinder. It will be seen thateach aperture of the mask and consequently each spot 72 is shaped as aparallelogram and that a line through the centres of the parallelogramsis oblique with respect to the direction of rotation of the cylinder. Itwill also be seen that one half of each parallelogram is in line, in thedirection of rotation, with the other half of the next parallelogram.Thus each track in the direction of rotation is scanned by two adjacentspots 72, one after the other. In addition, the light passing througheach of the parallelogram apertures will have a triangular intensityprofile when integrated in the direction of rotation and their tracksoverlap in the manner required by the above-mentioned specificationdisclosing overlapping areas of illumination.

The arrows 74 represent the delays which must be applied to theelectrical signals controlling the modulation of the beams to compensatefor the delay in the times at which different scanning beams cross thesame half-tone dot area.

Returning now to FIG. 2, each of the modulators 18 has a pair ofelectrodes for each light channel. Five of the electrode pairs areconnected through delay circuits D1 to D5 to the output of the screenedsignal generator 34, the other pair of electrodes being coupled directlyto the circuit 34. The circuit 34 receives the image-representing signalfrom a source 76, corresponding to the units 12, 20, 30 and 32 of FIG.1.

By rotating the prism 62 the angle of the line of scanning spots 72(FIG. 3) with respect to the direction 68 of cylinder rotation can beadjusted without altering the axis of light travel. The prism 62 isconveniently constructed by cementing together a 30°-60°-90° prism andan equilateral prism. The input and output surfaces should beanti-reflection coated, for example with a single layer of MgF₂optimised at the wavelength used. The base surface, at which theintermediate reflection occurs, should be silvered and protected and the"notch" surfaces should be polished.

The lens arrangement shown co-operates with the prism 62 to trim thespacing between scanning spots by adjusting magnification and thereby totrim the system to the delays applied to the modulating signals. In thesystem shown, the magnification setting using the lens arrangementaffects "delay" mainly and affects the "line-fit" slightly; rotation ofthe prism 62 mainly affects "line-fit" and slightly affects "delay". Thelens 66 images the mask at the required magnification (or, in practice,reduction) on the recording surface and lens 58 acts as a field lens andproduces a "telecentric" condition. The lens 58 is separated from themask by a distance which is about one tenth of its focal length, inwhich position it produces a magnification of about 1.1. By moving thelens a short distance, this magnification value can be varied slightlyand hence provides a fine setting of delay.

Although the apertures are elongated in the traverse direction, inpractice the resulting light will produce apparent elongation in thedirection of rotation, owing to the finite rise time of the modulatoramplifier drive signals, to give approximately equal resolution in thedirections of rotation and traverse. It will be appreciated that thelight profile also depends upon the distribution of illumination overthe mask aperture but this effect can then be corrected if necessary bythe mask shape.

The spacing of the beams in the direction of rotation prevents lightfrom two adjacent spots falling on a single area of the surface to betreated at the same time and thereby avoids interference effects whichwould otherwise result from the overlapping of coherent light beams.

Beam compression also reduces the spacing of the illuminated areas inthe direction of cylinder rotation and thereby reduces the requireddelays. A further advantage is that the reduction of the separation ofthe beam enables a smaller diameter lens 58 to be used, enabling a morecompact system, particularly if the lens 58 is in a turret ofalternative lenses. Such a turret permits easy change of screen ruling.

It is not essential to use delay circuits in the position shown in thedrawing; instead, the relevant information may simply be read from astore at the appropriate time.

Suitable electro-optic modulators can be obtained from Electro-OpticDevelopments Ltd. of 117, High Street, Brentwood, Essex, England underthe Type number PC 100/2 but other suitable Pockel and Kerr cellmodulator devices are available.

The lens 64 between lenses 58 and 56 provides the following advantages.Firstly, lenses 58 and 64 act as a telephoto combination and makepossible a shorter lens system. Secondly, by adjusting the lens 64 inconjunction with the lens 58, the magnification can be slightly changedwhile maintaining the telecentric condition. Thirdly, by insertingdifferent pairs of lenses 58 and 64, the magnification can be changedconsiderably, for example to produce different half-tone screen rulingswhile maintaining the telecentric condition (together with the finemagnification adjustment); a common focus position for lens 66 is alsomaintained.

If desired, a feedback system can be introduced to help control themodulator; the light to control the feedback system can be derived atthe mask 56, for example.

The beam splitter can be of any of the known kinds but is preferably ofthe kind disclosed in our copending Application No. 773,212, in the nameof John E. Aughton, filed Mar. 1, 1977. Briefly, this comprises a singleblock of light transmitting material having a pair of parallel sides,one of which is coated to enhance its ability for internal reflection,except at the end at which the light beam enters; the other of theparallel sides is coated in the direction of its length with asuccession of coatings so arranged that the parallel beams which emergefrom the differently coated areas of the latter side after internalreflection from the first side are all of substantially equal intensity.

We claim:
 1. A half-tone image recording system comprising: means forgenerating a number of separate beams of radiation which, in crosssection, form a line of spaced beams; a support for a recording surfaceto be treated by exposure to the beams of radiation; means producingrelative movement of the recording surface on the one hand and the beamson the other hand, whereby the recording surface is scanned by thebeams; modulating means for individually modulating the beams incidentat points on the recording surface in accordance with the image densityto be reproduced and with the screen density variations required toproduce a half tone image, the modulation being such that some or all ofthe modulated beams contribute towards a recording of a single half-toneimage dot; and, positioned in the beam path, means for rotating the lineof beams about an axis parallel to their direction of travel to alterthe angle of the line of beams incident on the recording surfacerelative to the direction of beam scanning over the recording surface.2. A half-tone image recording system comprising: a source of datarepresenting the densities of points in the image to be recorded; meansfor generating a number of separate beams of coherent radiation which,in cross section, form a line of spaced beams; a support for a recordingsurface to be treated by exposure to the beams of radiation; means forproducing relative movement of the recording surface on the one hand andthe beams on the other hand whereby the recording surface is scanned bythe beams; means for generating electric signals representing at anyinstant the densities of different points of a screen dot along a lineperpendicular to the scanning direction; means for individuallymodulating the beams; modulation control means for the individual beammodulation, responsive to the screen-representing signals and to theimage-representing signals such that at least some of the beams of aline of beams are modulated with a common image density derived from asingle image point; means positioned in the beam path for rotating theline of beams about an axis parallel to their direction of travel toalter the angle at which the line of beams is incident on the recordingsurface, the beams at the recording surface being spaced in thedirection of scanning as well as offset in a direction perpendicularthereto; and delaying means delaying the application of signals to theindividual modulating means in accordance with the spacing of thecorresponding beams in the scanning direction at the recording surface.3. A system in accordance with claim 1 in which the means for rotatingthe line of beams is a K prism.
 4. An image recording system inaccordance with claim 2, comprising a mask which includes aperturesthrough which the beams pass, the apertures having the shape requiredfor the beam spot at the recording surface, the system furthercomprising a lens system for adjusting the magnification of the maskimage at the recording surface.
 5. A system in accordance with claim 4,wherein the lens system comprises a positive lens followed by a negativelens.
 6. An image recording system in accordance with claim 4, in whichthe mask apertures and the location of their images at the recordingsurface are such that each part of the recording surface is in the trackof two successive scanning spots corresponding to two apertures of themask.